Method and system for determining the position and alignment of a surface of an object in relation to a laser beam

ABSTRACT

The present invention generally relates to a method and system for determining the position and alignment of a plane in relation to an intersecting axis and using that known position and alignment to allow for corrections to be made when using the plane as a reference plane. More particularly, the invention relates to a method and system for determining the angle of tilt of a planar surface in relation to a laser beam, and using the determined angle of tilt to calculate a correction factor to be applied to the laser beam. Briefly stated, the method and system ultimately calculates a correction factor, z-offset, that is applied when using the laser beam in a procedure.

1. PRIORITY

Priority as a continuation application is claimed to U.S. applicationSer. No. 10/269,340, now U.S. Pat. No. ______, filed Oct. 11, 2002. Thedisclosure of the priority application is incorporated herein byreference as if set forth in full.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention is laser focusing systems andmethods.

2. Background

Various laser procedures or operations require that a laser beam beproperly focused to a specific focal point. For example, in ophthalmiclaser surgery wherein eye tissue is to be photodisrupted or ablated inor on the tissue that is to be affected, the correct positioning of afocusing assembly used to focus a laser beam is very critical. Suchophthalmic surgical procedures include those in cornea, sclera, iris,the crystalline lens and related structures, vitreous, and retina, andfor treatment of glaucoma. Focal depth precision is also required inmany non-ophthalmic laser surgical procedures, such as applications indermatology and even “surgery” in DNA to excise portions of chromosomes.Also, non-biologic applications, such as photolithography andmicromachining require focal depth precision.

With presently used laser systems, however, it is a critical concernthat the object be positioned in a known relationship relative to thelaser system. For example, in eye surgery, it is only when the eye canbe positioned in a known relationship relative to the laser system thatthe laser beam can be directed to the desired area inside the eye with ahigh degree of accuracy. This is important because an inaccurately orimproperly directed laser beam could affect an area of the eye notdesired to be treated and cause permanent damage to the eye.

One way to accurately position the eye relative to a laser system forthe purposes of performing laser ophthalmic procedures is to use acontact lens to stabilize the eye. To do this, however, the alignment ofthe contact lens (glass plate or “aplanation lens”) relative to thelaser system must be known. As indicated above, if the lens alignmentrelative to the laser beam is not known, errors in accurate positioningof the laser beam can result.

In order to ensure that the alignment of a contact lens is knownrelative to a laser system, it is possible to permanently mount the lenson the laser system in a fixed orientation. If the contact lens is toremain mounted on the laser system, however, sterilization of the lensafter each laser ophthalmic procedure could be time consuming, difficultto accomplish and, most likely, very uneconomical. Alternatively, thecontact lens could be removed from the laser system, sterilized, andreplaced. Further, a disposable contact lens could be used for the laserophthalmic procedure. For either of these last two alternatives,however, the contact lens will require realignment with the laser systemafter the lens is mounted on the laser system.

U.S. Pat. No. 6,373,571 (incorporated herein by reference for allpurposes) issued to Juhasz et al., discloses a system and method foraligning an aplanation lens with a laser system. In particular, Juhaszdiscloses that in order to properly align the aplanation lens to a lasersystem, reference marks on the contact lens are brought into coincidencewith predetermined focal points along the laser beam paths. To this end,the laser system successively directs a laser beam along at least threepredetermined paths to respective predetermined focal points, and thecontact lens is positioned across these predetermined paths. Along eachpredetermined path, the laser beam is activated to establish a series oflaser marks on the contact lens. If the laser marks, predetermined focalpoints, and reference marks are all coincident, then the contact lens isproperly aligned with the laser system. If there is any displacementbetween any laser mark and reference mark, however, a retainer ringholding the aplanation lens is adjusted to align all reference markswith all predetermined focal points to align the lens to the lasersystem.

Because of the foregoing, it is however desirable to have alternativesystem and methods to determine the position and alignment of a plane ofan object in relation to an intersecting axis and using that knownposition and alignment to allow for corrections to be made when usingthe plane as a reference plane.

SUMMARY OF THE INVENTION

The present invention generally relates to a method and system fordetermining the position and alignment (including the angle andorientation of tilt) of a plane of an object in relation to anintersecting axis and using that known position and alignment to allowfor corrections to be made when using the plane as a reference plane.More particularly, the invention relates to a method and system fordetermining the position and alignment of a planar surface of an objectin relation to a laser beam, and using the determined position andalignment to calculate a correction factor to be applied to the laserbeam focal point. The method and system can also be adapted for objectswith curved surfaces. Briefly stated, the method and system ultimatelycalculates a correction factor, z-offset, that is applied when using thelaser beam in a procedure, such as photodisrupting corneal tissue belowan aplanation lens.

Once the position and alignment of the aplanation lens is determined,the positioning of the laser beam can be corrected to take the alignmentinto account when using the laser beam to photodisrupt corneal tissue.In general the method can be broken into two steps: first, determiningthe position and alignment of the aplanation lens relative to the laserbeam; second, determining the corrected position of the laser beamz-offset for later use in a procedure.

In one aspect of the inventive system, the movement of the focal pointof the laser beam is controlled by a CPU and software instructions. Thesoftware instructions may be contained on storage media such as CDs,hard drives, diskettes, or other electronic storage media devices.Additionally, the computer software (instruction sets) may be stored inROM, RAM or other storage devices capable of storing computerinstructions. A software program may be configured to capture the z-axislocation of the occurrence of detected plasma sparks. In addition to thez-axis location, the position of the x-axis and y-axis location may becaptured.

Various laser sources may be used with the inventive method and system,including infrared, visible, and UV lasers. Further, laser sources to beused with the inventive method and system may be continuous wave,Q-switched pulse, and mode-locked ultrashort pulse lasers. Although theforegoing is not an exhaustive list, lasers of the foregoing type may beused with the present invention. In one aspect of the invention thelaser beam is formed of a continuously repeating train of short opticalpulses in the range of femtoseconds or picoseconds. In one embodiment,the laser source is an infrared ultrashort pulse laser with a pulseduration of less than 10 picoseconds. While various laser sources may beutilized, in one femtosecond laser system, the laser energy per pulse tophotodisrupt the object and create a plasma spark is about 1-5 μJ for afocus of 2.5 μm.

The object used with the present invention is a material capable ofproducing a detectable plasma spark when contacted with the focal pointof a laser beam. Some materials where a plasma spark may be createdinclude glass, silicon, or plastic (including medical grade plastic),and biologic materials. The object is either permanently or temporarilyaffixed to the laser system such that the object falls within the pathof the laser beam. A cage, base, frame, or other holding device may beused to position the object in place. For example, an aplanation lenscomposed of highly purified fused silica is placed in a cone shapedframe which is connected to the laser system as described in co-pendingU.S. application Ser. Nos. 09/772,539 (Publication No. US2002/0103481)and 09/896,429 (Publication No. US2002/0103482) (the disclosures ofwhich are incorporated herein for all purposes). Another example is amicroscope slide positioned in place by using pressure to hold the slidein place.

In one aspect of the invention, there is a method and system fordetermining the occurrence of a plasma spark about the surface of anobject, or within the object. The method and system utilizes aphotodetector to detect the occurrence of the plasma spark when thefocal point of the laser beam contacts the surface of the object, orwhen the laser beam is focused within the object. The photodetectoridentifies when a plasma spark occurs. The photodetector may be any oneof a photodiode, CCD, photomultiplier, phototransistor, or any devicesuited for detecting the occurrence of a plasma spark.

In one aspect of the invention, there is a method and system fordetermining the position and alignment of a surface of an object inrelation to a laser beam. A laser system for generating a laser beam andan object having a substantially planar surface are provided. The methodand system may also be adapted for objects with a curved surface. Theobject is positioned in the path of the laser beam. The object may bepermanently or temporarily affixed to the laser system. The focal pointof the laser beam is repeatedly moved along a predetermined pattern in aplane perpendicular to a z-axis of the laser beam. Plasma sparks aredetected when the laser beam focal point contacts the object. Theposition and alignment of the surface of the object in relation to thelaser beam is determined.

In one aspect of the invention, moving the focal point of the laser beamincludes starting at a starting point on a z-axis plane such that thefocus of the laser beam is not in contact with the object; repeatedlymoving the focal point of the laser beam along a predetermined patternin at least one plane perpendicular to the z-axis; and after anoccurrence of the completion of movement of the laser beam along thepredetermined pattern, repositioning the focal point of the laser beamon the z-axis a set distance Δz from the previous z-axis location. Thepredetermined pattern is preferably circular in shape. In oneembodiment, the focal point of the laser beam may be positioned belowthe object and the laser beam moved up towards the object. Or in anotherembodiment, the focal point of the laser beam may be focused somewherebetween the laser source and the object, and the laser beam movedtowards (or downward) to the object.

In another aspect of the invention, detection of plasma sparks includesidentifying a first plasma spark when the laser beam comes into contactwith the object; recording a first z-axis location of the first plasmaspark; identifying the completion of the predetermined pattern byidentifying a second plasma spark along the complete predeterminedpattern; and recording a second z-axis location of the second plasmaspark.

Further to detecting the plasma sparks, the position and alignment ofthe object in relation to the z-axis using the first z-axis location andthe second z-axis location is calculated. In one embodiment, calculationof the tilt angle (alignment) of a surface of an object is performed byutilizing the formula θ=tan⁻¹(Δz/D), where Δz is the difference betweenthe first z-axis location and the second z-axis location, and D is thediameter of the predetermined pattern.

In one embodiment of the invention, plasma sparks are visually detectedby the operator. The occurrence of a first plasma spark and theoccurrence of a second plasma spark at the completion of a predeterminedpattern are detected. An input device such as a foot switchinterconnected with the laser system is manually operated. When theoperator of the laser system visually identifies the first occurrence ofa plasma spark, then the input device is triggered to signal to thecomputer to record the first z-axis position. The laser focal pointcontinues through the object in iterative predetermined patterns. Whenthe operator of the laser system visually identifies the completion ofthe predetermined pattern, then the operator actuates the input device,which in turn triggers the computer to record the second z-axisposition.

In another embodiment of the invention, the detection of the plasmaspark includes providing a photodetector for detecting plasma sparks,and identifying the occurrence of the plasma spark with thephotodetector. The photodetector may be any one of a photodiode, CCD,photomultiplier, phototransistor, or any device suited for detecting theoccurrence of a plasma spark.

In one embodiment of the invention, the detection of the plasma sparkincludes providing a video camera for taking images of the object andcapturing a series of images of the object. The position and alignmentof the surface can be determined by subtracting the pixels of a previousimage from the pixels of a current image and subsequently adding all theresulting pixels that exceed a certain threshold to become a finalnumber for that image which correlates with the plasma intensity forthat image. The final number for each calculation may be plotted on agraph to establish a plasma intensity curve.

The step of determining the alignment of the aplanation lens relative tothe laser beam can be broken into several substeps, as follows. First,if the z-axis is defined as the path of the laser beam, the focal pointof the laser is directed on the z-axis below the aplanation lens, at apoint z₀. The focal point of the laser beam is then moved along a closedpattern, for example, a circle with a fixed diameter less than thediameter of the aplanation lens, in a plane perpendicular to the z-axis.After the focal point has completed the closed pattern, the focal pointis adjusted at a set distance (also referred to as a separation layer),z_(x), above to z₁, and the moving step is repeated. These last twosteps, adjusting the focal point up the z-axis to Z₂ and moving thefocal point in the closed pattern, are repeated i times until the focalpoint of the laser is adjusted up the z-axis to z_(i) and the focalpoint makes contact with the aplanation lens, causing a plasma spark.When this occurs, the position of the focal point, z_(i) is recorded.The focal point is then adjusted z_(x) above the previous starting pointand the focal point is moved along the closed pattern in a planeperpendicular to the z-axis until the laser makes contact with theaplanation lens along the entire closed pattern, causing a plasma sparkalong the entire closed pattern. When this occurs, the position of thefocal point, z_(j), is again recorded. A Δz can be determined, bycalculating the distance between z₀ and z_(j). Using the diameter of theclosed pattern and the total distance along the z-axis the focal pointtraveled, trigonometry can be used to determine the angle, θ, of theaplanation lens relative to the z-axis.

In one aspect of the invention, a method and system for determining thealignment of a surface of an object in relation to a laser beam isdisclosed. An object having a substantially planar surface is provided.A laser system for generating a laser beam is utilized to create atleast three plasma sparks at the surface of the object. The laser systemhas a CPU with software configured to carry out the process andcomputations. The plasma sparks may be detected in any manner, includingthose described previously, such as manually/visually, a photodetector,or the video image analysis. By detecting three points about the planarsurface of the object, it is possible to identify a plane in relation toa z-axis of the laser beam and the plane's tilt relative to the laserbeam z-axis. Additionally, the curvature of a surface may be detected ifthe surface is not planar. In this case, multiple points would beidentified with plasma sparks and their x-,y-,z-coordinates recorded.The curvature of the surface may then be computed.

In one aspect of the invention, a method and system for determining afocal point of a laser beam upon an object having a substantially planarsurface is disclosed. The novel system and method utilizes aninterferometer to determine a fringe pattern of a reflection of a laserbeam from the object. In this particular system and method, an objecthaving a substantially planar surface is provided. A laser system forgenerating a laser beam is provided. The laser system has a centralprocessing unit configured for instructing movement of the laser beam.The interferometer is interconnected with the laser system. The laserbeam is focused at or near the substantially planar surface. The laserbeam is reflected back from the planar surface. A fringe pattern isdetected. Based on the analysis of the fringe pattern, the laser beam isdetermined to be in or out of focus. A software program for execution onthe central processing unit may be configured for focusing the laserbeam at or near the substantially planar surface of the object,detecting a fringe pattern of the laser beam, and determining whetherthe laser beam is in focus based on the fringe pattern. If the fringepattern lines are substantially parallel to one another, then the laserbeam is focused on the planar surface.

In yet another aspect of the invention, another method and system fordetermining a focal point of a laser beam upon an object having asubstantially planar surface is disclosed. The laser system computermonitors the dependence of the signal on depth. Change in the signalindicates the interface between the lower surfaces of the aplanationglass and the cornea. A laser system for generating a laser beam has acentral processing unit configured for instructing movement of the laserbeam. A photomultiplier with a band pass filter for detecting anonlinear frequency signal generated by the laser beam is interconnectedwith the laser system. A software program for execution on the centralprocessing unit is configured for monitoring a nonlinear frequencysignal generated by the laser beam, and determining whether the laserbeam is in focus. The nonlinear frequency signal may be any one ofsecond harmonic generation, third harmonic generation, stimulated Raman,or white light generation and others.

In yet another aspect of the invention, a method and system fordetermining the distance between two objects is disclosed. A lasersystem for generating a laser beam having a central processing unitconfigured for instructing movement of the laser beam is utilized tocreate and detect a first plasma spark at the surface of a first object,and to create and detect a second plasma spark at the surface of asecond object. A software program is configured for identifying a firstpoint at the outer surface of a first object by detecting the occurrenceof a first plasma spark; identifying a second point at the outer surfaceof the second object by detecting the occurrence of a second plasmaspark; and determining the distance between the first point and thesecond point. The software program records the x-,y, z-axis location ofthe first and second points, and then calculates the distance betweenthe points. The detection of the plasma spark may be done by any devicecapable of detecting a plasma spark. In one embodiment, the plasma sparkis detected by a photodetector. Some examples of a photodetectorincludeany one of a photodiode, CCD, photomultiplier, phototransistor,or any device suited for detecting the occurrence of a plasma spark.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a perspective view of the system used to determine positionand alignment of the aplanation lens relative to the laser systemillustrating an embodiment of the present invention;

FIG. 2 is a schematic view of the aplanation lens and the laser beam;

FIG. 3 is a flowchart illustrating a method for determining theposition, alignment, and orientation of the aplanation lens relative tothe focal plane of the laser beam;

FIG. 4 is a graph illustrating a video image analysis for determiningthe position, alignment, and orientation of an aplanation lens relativeto laser beam;

FIG. 5A-5C are drawings illustrating detected pattern fringes whileusing an interferometer for focusing a laser beam; and

FIG. 6 is a graph illustrating dependence of second harmonic signal onbeam waist position in pig eye where the positive sign on the Depth axiscorresponds to the position inside the cornea and the zero positioncorresponds to the cornea-glass interface.

DETAILED DESCRIPTION OF THE INVENTION

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

Referring now to FIG. 1, a schematic view of one embodiment of anaplanation lens position and alignment system according to the presentinvention is depicted. The major components of the system 10 are a lasersystem 12 and an aplanation lens 14. To accomplish laser ophthalmicsurgery, the laser system 12 includes a laser source 16 which is mountedon the system housing (not shown). This laser source 16 generates alaser beam 20 from an origination point 22, as shown in FIG. 1. In oneembodiment of the invention, the laser beam 20 has a pulse duration lessthan three hundred picoseconds (<300 ps) and a wavelength of betweenapproximately 400-3000 nm. Preferably, the laser operates at 1053 nm,with a pulse duration of approximately 600-800 femtoseconds, and arepetition rate of 10 kHz. FIG. 1 shows that the laser beam 20 is usedto define a z-axis 24 that is parallel to the path of the laser beam. Asdiscussed herein, the inventive system and method are shown through theuse of an aplanation lens. However, the position and alignment of otherobjects may be determined. Thus, the inventive system and method shouldbe construed to cover any other object for which one wants to determineits position and alignment in relation to a laser beam.

Determination of Object Alignment

Referring to FIG. 2, a schematic view of a laser beam 20 and tiltedaplanation lens 14 is shown. To determine the position and alignment ofthe aplanation lens 14 in relation to the z-axis 24 of the laser beam,the focal point of the laser beam is first directed to a point on thez-axis 24 that is below the aplanation lens 14. This first point isreferred to as z₀ 26. The focal point of the laser beam is then movedalong a closed pattern. The closed pattern is a shape where the laserbeam focal point will travel. As the laser beam focal point travelsalong the closed pattern, the laser beam is fired. A spot distance ofthe laser beam may be set by the laser system such that the laser beamis fired on the closed pattern for a particular distance. For example,in one embodiment, the spot distance may be set to 1 μm-30 μm. For aparticular object and laser source being utilized, the spot distance maybe different than the aforementioned example.

In a preferred embodiment, the closed pattern is a circular shape havinga diameter (“D”) 28. The closed pattern is made in a plane perpendicularto the z-axis 24. For an ophthalmic procedure using the aplanation lens,the closed pattern should have a diameter sufficiently wide, such thatafter the position of the aplanation lens and alignment determination iscompleted, a cornea then pressed against the aplanation lens does notcontact an area of the closed pattern. In certain tests using anaplanation lens, a 7-9.5 mm diameter was utilized for the closed patternand was found sufficiently wide. Other diameters of course may beutilized depending on the type of procedure and the particular objectfor which alignment is being determined.

After the first closed pattern is completed, the focal point of thelaser beam is then adjusted up the z-axis 24 a set distance z_(x) 30 toanother starting point z₁ 32 where z₁=z₀+z_(x). The value for the z_(x)distance between each successive closed pattern is also referred as aseparation layer distance. For each pass of the closed pattern, thelaser beam focal point will move a distance along the z-axis based onthe separation layer setting.

The focal point of the laser beam is then again moved along a similarclosed pattern in a plane perpendicular to the z-axis 24 and thenadjusted up the z-axis to z₂ 34 where Z₂=z₁+z_(x). The steps of movingthe focal point along the closed pattern and adjusting the startingpoint of the focal point of the laser beam up the z-axis 24 are repeatedn times, until the focal point of the laser along the closed patternmakes contact with the aplanation lens 14, causing a first plasma spark,at z_(n) 36, which may be detected. The particular manner in which theplasma sparks are detected is described below.

A CPU utilizing software preferably instructs the movement of the focalpoint of the laser beam. While moving the laser beam, the software mayrecord the coordinates of the focal point. For example, as the closedpattern is followed, the specific x-, y- and z-coordinates of the laserbeam focal point will be known. This is true because it is the softwareinstructing the movement of the focal point through the closed patternat particular coordinates. Thus, the laser system software may beconfigured or programmed to record the x, y, and/or z-coordinates basedon certain defined events.

The particular z_(n) when the first plasma spark occurs is recorded. Thesteps of moving the focal point along the closed pattern and adjustingthe starting point of the focal point of the laser beam up the z-axis 24are repeated m times, until the focal point of the laser contacts theaplanation lens 14 along the entire closed pattern, causing a plasmaspark along the entire closed pattern, at z_(j) 38, where j=m+n, whichis detected. The point z_(j) is recorded. The particular manner in whichdetection of the completion of the closed pattern occurs is laterdescribed below.

For a better understanding of the inventive method, FIG. 3 sets out inflowchart form certain steps of the present invention. In step 201, thefocal point of the laser beam is set at a point on the z-axis below theaplanation lens, z₀. Next in step 202, the focal point of the laser beamis moved along a pattern, preferably in the shape of a circle having adiameter D, in a plane perpendicular to the z-axis. During the movementof the laser beam along the pattern, a check is made for the occurrenceof a plasma spark. If a plasma spark is detected, then in Step 204, thez_(n) location is recorded. Likewise, the x_(n) and y_(n) coordinatesmay also be recorded. If no spark is detected, when the pattern iscomplete, then in Step 203 the focal point of the laser beam is moved upthe z-axis a determined distance, z_(x). Step 202 is repeated until aplasma spark is detected.

In Step 205, the focal point of the laser beam is moved up the z-axis adetermined distance, z_(x). Then in Step 206, the focal point of thelaser beam is moved along a predetermined pattern, preferably in theshape of a circle having a diameter D, in a plane perpendicular to thez-axis. During the movement of the laser beam along the pattern, a checkis made for the occurrence of a completion of a plasma spark for thecircumference of the circle. If a completion of the entire circle isdetected, then in Step 207, the z_(j) location is record. Also, thelocation of the x_(n) and y_(n) coordinates may also be recorded. If thecompletion of the plasma spark for the circumference of the circle isnot completed, then Step 205 repeats. Lastly, in Step 208, the tilt ofthe aplanation lens can be determined.

Visual Detection of Plasma Spark

The plasma spark may be visually detected by the operator. For example,a foot switch operated by the user of the laser system may identify whenthe plasma spark occurs. The movement of the focal point along theclosed pattern is performed as discussed above. When the user firstdetects the plasma spark, a foot switch may be activated. The activationof the switch signals the computer to record the z-axis coordinate ofthe first plasma spark. When the user detects completion of the closedpattern by watching a complete plasma spark along the closed pattern,the user activates the foot switch again. Thus, the second z-axiscoordinate is obtained. With both coordinates the tilt of the lens maythen be determined.

Electronic Detection of Plasma Spark

In another embodiment, a photodetector connected with the laser systemmay be utilized to detect the occurrence of plasma sparks. Thephotodetector can be any device capable of determining such an event.For example, a photodetector may include a photodiode, CCD,photomultiplier, phototransistor, or any device suited for detecting theoccurrence of a plasma spark.

The photodetector can be utilized to determine a first occurrence of theplasma spark and the completion of the closed pattern, thereby givingfirst and second z-axis coordinates which then may be used to calculatethe tilt of the aplanation lens.

In one embodiment, a photodetector is connected with the laser system.The photodetector is placed in a position on, adjacent to, or near thelaser system where the photodetector can detect the plasma spark. Thephotodetector generates a voltage or signal when the laser beam createsa plasma spark in the aplanation lens. When the photodetector firstdetects a plasma spark, then the laser system software records the firstz-axis coordinate.

For the second z-axis position at the completion of the plasma sparkalong all of the closed pattern, the identification of the completionmay be determined in different ways. One way to determine the completionof the closed pattern is to evaluate the voltage or signal from thephotodetector and compare it with a known time for completion of theclosed pattern. The laser system software may be configured to calculatethe duration of time necessary to complete a given closed pattern. Atthe completion of the closed pattern, the voltage or signal of thephotodetector can be evaluated. If the voltage or signal of thephotodetector indicates that a plasma spark is occurring at the end ofthe closed pattern, then we know that a plasma spark has occurred at theend of the closed pattern. With this known completion point, then thesecond z-axis position can be determined.

Information about the orientation of tilt can be obtained by determiningthe x-y coordinate where the most intense plasma spark is detectedwithin the object. The strongest signals from the plasma sparkcorrespond to the deepest position within the object.

Video Image Detection of Plasma Spark

In an alternative embodiment, a video camera is utilized to captureimages of the aplanation lens as plasma sparks are being generated. Bycomparing sequences of captured images, it is then possible to use theimage information to determine the tilt of the aplanation lens. In oneembodiment, an NTSC camera with a rate of 30 frames per second wasutilized. However, other video cameras with different frame rates may beutilized.

In general, video images are searched for plasma spark as the laser beamfocal point is scanned upwards toward the bottom surface of theaplanation lens. Similar to the visual/manual and photodetector methodsdescribed above, the laser beam focal point is set at a beginning pointsuch that the focal point of the laser beam does not create a plasmaspark. The laser beam focal point is then moved through successiveclosed patterns whilst first and second z-axis coordinates aredetermined.

In one embodiment, 8-bit grey scale images are captured and evaluated. Agrey scale image has pixels with a grey scale value between 0 (black)and 255 (white). The grey scale values ranging between 0-255 indicatesthe brightness for a particular pixel. For example, if an area ofcertain pixels of an image had a value of zero or near zero, this wouldindicate that portion of the image was dark. If an area of certainpixels had a value of 255 or near 255, this would indicate that portionof the image was very light. Thus the higher the number for the pixelsof a certain area of an image, the brighter (or whiter) that area wouldbe. Based on this pixel valuation model, the occurrence of a plasmaspark can be detected. When a plasma spark occurs and an image is taken,more higher-ranging pixels would exist than would exist if the plasmaspark was not occurring. This is because the plasma spark creates a verybright light that would be noted in the image.

Referring now to FIG. 4, a graph is shown illustrating an aplanationlens tilt determination utilizing the iterative image comparison method.The frequency of image frames to be captured is set at a periodic timeinterval. The x-axis on the graph represents the frame number of acaptured video image. In the illustrated example, a focal point of thelaser beam was set in a circular pattern with a diameter of 7.8 mm. Thespot distance of the laser was set at 3 μm. An energy level of 3 μJenergy for the laser source was utilized. The y-axis on the graphrepresents the Total Compared Image Value, for those pixels above acertain threshold number. In the experiment, the threshold number wasset at a value of 20.

The plasma spark line 60 shows the processing of several frames ofimages before, during and after the occurrence of plasma sparks. Thevideo image process begins with the capture of a first video image.After a preset time interval, the next image is captured. The firstvideo image and the second video image are then compared to one another.

Each pixel value (0-255) from the first image is added together toobtain a first image value. Also, each pixel value (0-255) from thesecond image is added together to obtain a second image value. If athreshold value is set, then only those pixel values having a valuehigher than the threshold value would be added together. Utilizing athreshold value reduces the light noise dramatically and allows theprocess to run at full room light and high illumination of theaplanation lens.

The first image value is subtracted from the second image value giving aTotal Compared Image Value. The Total Compared Image Value, which isstored in memory of the CPU, may be plotted on a graph. Although notshown on the graph, for a Total Compared Image Value, the laser systemsoftware would also know or have stored the x-,y-, and z-coordinates forthe particular image frame. Thus, for a particular Total Compared ImageValue, the x-, y-, and z-coordinates may be associated with theparticular Total Compared Image Value.

As illustrated in FIG. 4, prior to about frame 860, no plasma spark hasoccurred. On the y-axis, the plasma spark line is shown as a linear linehaving a Total Compared Image Value of zero. During the process theambient light is preferably maintained at a consistent level. As shownin FIG. 4, literally no noise signal exists before the plasma starts,even at full room light. As the plasma spark starts, from about frame860, the increasing mountains of signals occur as is shown on plasmaspark line 60.

The spacing between each side of a mountain on the plasma spark line 60represents the completion of one full circle. The first mountain 64indicates the first occurrence of a plasma spark. The exact x-ycoordinates at any mountain top gives the tilt axis. The first time themountain does not go down to 0 (or some low threshold), the plasmacircle is completed (second or final contact).

To more easily detect the first and the second contact, the plasma sparkline 60 is further processed in the following way. A binary signal (orplasma spark state) may be created with the following process. Thebinary signal or plasma spark state is set to one 1 if the TotalCompared Image Value is over a certain value. If for a particular imageframe, the Total Compared Image Value is greater than the set value (inthe example it was set to 1), then for that frame the plasma spark statewould be set to 1 or True. If the Total Compared Image Value is belowthe set value, then the plasma spark state would be set to 0 or False.In this manner, as shown on the graphed plasma spark state line 62, thestate of the plasma spark for a particular image frame and time would beknown.

The distance between two consecutive mountain peaks is equivalent to thelayer separation parameter defined by the laser software. This isusually in the order of 2-10 micrometers but may vary according to thedesired accuracy. For each mountain peak, the closed pattern makes onerevolution and for each revolution, the focus position moves upward inthe z-direction in the amount of the layer separation. The amount ofpeaks contained between the first plasma spark 64 and the full closureof the pattern 61 determines the following Δz=|z_((1st plasma))−z_((Full closure))|. The determination of the tilt axisis dependent on the position of the x-y coordinate at the time themountain peak is present. An axis line can be drawn 180° from the x-yposition of the mountain peak, relative to the center of the circularpattern. The determination of tilt is as follows θ=tan⁻¹(Δz/D) where Δzis the difference of z position between the first plasma spark 64 andthe full closure of the pattern 61 as detected by the CCD camera and Dis the diameter of the circular pattern.

Calculation of Tilt of the Lens and Z-Coordinate Offset

The alignment of the aplanation lens 14 in relation to the z-axis 24 isthen calculated by using the following equation: θ=tan⁻¹(Δz/D); where θ40 is the angle between the aplanation lens 14 and a plane perpendicularto the z-axis 24, and wherein Δz is the difference between the firstz-axis location and the second z-axis location, and D is the diameter ofthe predetermined pattern. The angle φ42 between the z-axis 24 and theaplanation lens 14 is 90-θ.

Although the methods above discuss obtaining a second z-axis location byelectronic or manual means, the second z-axis may be calculated. Afterthe first z-axis location is found, then the second z-axis iscalculated. The second z-axis location would be the point on a circularpredetermined pattern opposite the first z-axis location. This holdstrue since, by using a circular predetermined pattern, the first z-axislocation is the lowest point of the tilt (if scanning the laser from thebelow the aplanation lens upwards) and the highest point would always bethe point on the predetermined pattern opposite the first z-axislocation. Thus, the first z-axis location may be determined (along withthe x-,y-coordinates) and then using the known diameter of the circularpattern, the second z-axis location may be determined.

Utilizing the circular predetermined pattern, by finding the first andsecond z-axis location, the plane of the contact surface of theaplanation lens can be determined along with the orientation of theplane about the z-axis.

Determining the tilt of the aplanation lens 14 in relation to the laserbeam is very useful. In the field of ophthalmic surgery, a more precisephotodisruption of tissue of the eye can be achieved. For example, it isimportant in ophthalmic laser surgery procedures that photodisruption bevery precise. Whilst utilizing an aplanation lens, a patient's cornea ispressed against the lens, thereby flattening the cornea against theglass surface of the lens. With a perfectly calibrated laser system,using a perfectly formed aplanation lens, the contact surface (thecontact plane) of the aplanation lens would be perpendicular to thelaser beam. This would allow the focusing of the laser beam at az-coordinate in the cornea in one x-y location to be the samez-coordinate if the laser focus was moved to another x-y location. Butif the aplanation lens were tilted, this would cause the focus of thelaser at one x-y location in the tissue of eye to actually be differentthan another x-y location in the tissue of the eye. But if the tilt ofthe aplanation lens were known, then the z-coordinate (or focal depth)for a particular x-y location could be offset or compensated for to takeinto consideration the lens tilt.

Three-Point Method to Determine Tilt of an Object

An alternative way to determine the tilt of a surface of an object inrelation to a z-axis of a laser beam is to determine three points of anobject. A laser beam may be focused at a z-axis point such that thefocal point of the laser beam does not contact the object. This may beat any x-,y-coordinate. The laser beam z-axis focal point isincrementally moved a specified distance and the laser fired. The focalpoint is moved again a set distance and fired. This continues until afirst plasma spark is detected. The detection may be by any manner,including the method described above, manually, via photodetector, andvideo image comparison. The first point (its x-,y-, and z-coordinates)is recorded or saved in memory or storage by the laser system.

The laser system then directs the laser beam to a second x,y-coordinate.The focal point of the laser is then moved to a z-axis point such thatthe focal point of the laser beam does not contact the object. Thenagain, the laser beam z-axis focal point is incrementally moved aspecified distance and the laser fired. This continues until a secondplasma spark is detected. The second point (its x-,y-, andz-coordinates) is recorded or saved in memory or storage by the lasersystem.

The laser system then directs the laser beam to a third x,y-coordinate.The focal point of the laser beam is then moved to a z-axis point suchthat the focal point of the laser beam does not contact the object. Thenagain, the laser beam z-axis focal point is incrementally moved aspecified distance and the laser fired. This continues until thirdplasma spark is detected. The third point (its x-,y-, and z-coordinates)is recorded or saved in memory or storage by the laser system.

Having now determined three surface points of a surface of the object, aplane of the surface in relation to a z-axis of the laser be would beknown. Knowing the plane of the object, then subsequent procedures canuse the plane as a reference plane for z-offset.

Also, the distance between two points may be calculated by detecting afirst plasma spark at the surface of a first object, and detecting asecond plasma spark at the surface of a second object. The detection ofthe first and second plasma spark may be detected by the methodsdescribed above. The z-axis coordinate of each plasma spark is then usedthe determined the distance between the surface of each object where theplasma spark is detected.

Z-offset and Gain Calibration Procedure

By determining the alignment (or tilt) of a surface of an object inrelation to a laser beam (or z-axis of the laser beam), a z-offset valuemay be utilized for subsequent laser system operations. For a particularx-,y-coordinate, the z-coordinate may be offset a particular distance toallow the focus of the laser beam to be at a plane parallel to the planeof the tilt of the object.

In one embodiment, a software program commands a displacement of afocusing assembly of a laser system by writing a voltage to aDigital/Analog card. A z-Galvo will in turn move the focusing assemblyto the desired focal depth position based upon the commanded voltage bydirecting a current to the motor-driven focusing assembly. A linearencoder positioned within the laser system senses the linear movement ofthe focusing assembly. An intelligent controller interoperating with thehost computer and software program utilizes a sensor to read an encoderstrip attached to the focusing assembly. As the lens is moved intoposition, encoder feedback is provided by an intelligent controller andan actual focusing assembly position is obtained.

To measure the z-gain, a second point needs to be measured. Measurementof the z-gain may be achieved by utilizing a second object, such asglass that has a substantially planar top and bottom surface that aresubstantially parallel to one another.

In one experiment, a 160 μm thick microscope slide was mounted againstthe contact glass of the aplanation lens contact plane. The slide wasmade out of borosilicate glass (Corning 0211) with a refractive index of1.521 at 1060 nm. The flatness of the slide was measured. It hadparallel top and bottom planar surface within ±1 μm over the whole slide(22×22 mm). The slide is pressed against the contact glass by slightlypushing from below with a rod and a round plastic screw head on top ofit. This results in an air gap below the slide at the circle diameter ofthe closed pattern. The circular closed patterns are now cut like in thez-offset procedure except that the starting depth is set at 200 μm. Thissimulates focusing the laser beam into the corneal tissue. To correctfor the human cornea (n=1.377), the 160 μm thick borosilicate glasscorresponds to a 146 μm thick cornea layer. This was simulated with theWinLase™ 3.0 Pro software using a Gaussian beam with an f#=1.48 focusingnumber of the objective lens.

With the correction in place, the software is expected to report anoffset of 146 μm if the z-offset was zeroed before a procedure. If thenumber is off, then the z-scale factor (z-gain) is off by the followingamount: New z-scale factor=(146 μm/measured offset)*old z-scale factor

After correcting the z-scale factor in the laser system settings, thez-offset needs to be redone because it might not fall together with aO-voltage on the z-scanner and therefore can be affected by a gainchange.

Interferometric Laser Focus Detection

Another way to measure the position of a surface of an object relativeto a laser beam is utilizing an interferometer. After measurement, thelaser system may then account for variances of height dimensions of theobject and set offset parameters for the focal depth. Offset parametersin software allow canceling the effect of variances of height dimensionsof the aplanation lens, thereby delivering consistent surgical depths.

This method utilizes the curvature of the wave front of a laserreflected back from the glass surface of the aplanation lens. Thecurvature of the wave front is measured by an interferometer.

There are two ways to relate fringe curvatures to focal depth. First, byknowing the geometry of the optics and the interferometer, the fringepatterns can be exactly calculated and related to focal depths. Howeverthis method would require a precise knowledge of the beam geometry.

A second, more practical method is to calibrate the machine tomeasurable focal positions. This is the approach we followed with ourimplementation. In one implementation the machine is set to cut patternsin a glass sample at different depths while the interference patternsare simultaneously recorded. Then the cutting depths in the sample aremeasured with the help of a microscope and related to the curvatures ofthe fringes as previously recorded.

The interferometer utilizes a reference beam, which is split directlyfrom the laser beam before entering the delivery system, and a measuredbeam, which passes through the delivery system. The reference beam isessentially a parallel beam. The measured beam is part of the laser beamthat reflects back from the optical surface of the aplanation lens. Thereflected beam retraces the optical path through the laser focusingoptics and the scanner system in a backward direction.

If the reflecting surface is at the focal point, then the back-reflectedbeam retraces the same path all the way through the delivery system andleaves it as a parallel beam. This beam can be interfered with areference beam. In this case, both beams are parallel and they make aninterference pattern with straight fringes. On the other hand, if theaplanation lens is out of focus, then the back-reflected beam does nottrace the very same path backwards, and it leaves the delivery system asa convergent or divergent beam. Convergent or divergent beams combinedwith parallel beams produce curved fringe patterns. The positioninformation of the focus can be extracted from the interference pattern,essentially from the sign and magnitude of the curvature of the fringes.

In one embodiment an image processing method is followed. A raw image isfirst captured then filtered and enhanced by convoluting the image witha spatially periodic kernel. This process smoothes imperfections of theimage which are of random nature for example due to dust particles onthe optics. At the same time the spatial periodicity of the kernelenhances the contrast of the interference pattern with the rightperiodicity.

The next step of the image processing is edge detection by Canny EdgeDetection algorithm. (Canny, A. (1986) A computational approach to edgedetection. IEEE Trans. PAMI, 8:769-698.)

The edge fragments are then analyzed. Fragments shorter than a givenlength are discarded. The longer fragments are fitted with a polynomialcurve. The second order coefficient of the polynomial gives thecurvatures of the individual fringes. Finally curvatures from individualfringes are averaged.

In one embodiment, the interference pattern is captured by a videocamera and frame capture software described above. The pattern may beanalyzed by computer software. The curvature of the fringe pattern isextracted and the focal position calculated. To determine the focalposition, when the fringe pattern has parallel beams, then the laserbeam is focused. One way to determine how much the laser beam is out offocus, is the mass calibrate various curvatures of the fringe patternand measure the focal distance. For example, a micrometer may be used todetermine the various focal distance for a particular fringe curvature.For a particular fringe curvature, a focal depth value may be stored ina table. Then for subsequent uses of the laser system, a particularfringe pattern curvature, may be determined and then looked up in thetable to determine the focal position. Alternatively, for the curvaturebehavior could be evaluated to determine an algorithm, such that for aparticular fringe curve a focal position could be derived.

Various experiments were performed to determine the fringe patterns andthe relation to the focus of the laser beam. In one experiment, themeasured interference fringe pattern curved downwards. This is shown inFIG. 5A. The focus of the laser beam was found to be 20 μm above thecontact plane of the aplanation lens. In another experiment, themeasured interference fringe pattern formed straight lines. This isshown in FIG. 5B. The focus of the laser beam was found to be on theglass surface of the aplanation lens. In a third experiment, themeasured fringe pattern curved upwards. This is shown in FIG. 5C. Thefocus of the laser beam was found to be 10 μm below the contact plane ofthe aplanation lens.

Measuring one point at the optical center of the field of view of theaplanation lens provides a z-offset number. This method may be used tomeasure three point measurements of the contact plane of the aplanationlens to determine the tilt of the focal plane.

This interferometric method not only has the advantage of determiningthe focal point of a plane of an aplanation lens, but also may be usedto detect laser beam distortions. Some of these distortions may be i)inherent to the design of the laser system optics, such as spherical andchromatic aberrations, ii) coming from the laser, such as spatial chirp,iii) distortions from component level aberrations, such as out of specmirror flatness, or iv) distortions due to system misalignment.

If the measured focal position of the laser is outside of apre-determined acceptable range, the laser system software may beconfigured to instruct the servo system to modify offset values for thez-axis focal position and then bring the laser system into an acceptablerange. Also, the software parameters for a surgical pattern may beconfigured to accommodate hardware offset and tilt of the laser focalplane relative to a surgical plane.

Nonlinear Frequency Conversion

Another method to determine the depth of focus of a laser beam isutilizing a photo multiplier with band pass filter to monitor thenonlinear frequency signal generated by laser beam. The laser systemcomputer monitors the dependence of the signal on depth of focal point.Change in the signal indicates the interface between the lower surfacesof the aplanation glass and the cornea. Nonlinear frequency conversionmethod is noninvasive. The depth calibration can be performed while theaplanation lens is docked on a patient's eye thus reducing the errorintroduced by mechanical backlashes.

This method is based on usage of different nonlinear effects in glassand the cornea to generate light at frequencies other than the frequencyof the laser beam. The effects can include, but not be limited to,second harmonic generation, third harmonic generation, stimulated Raman,white light generation and others. At laser beam intensities close tophotodisruption threshold, conversion efficiencies of mentionednonlinear processes are high enough to generate a detectable signal.These signals have quadratic or higher order dependence on inputintensity and will be confined in space to the beam waist and willtherefore increase the accuracy of interface detection.

A photo multiplier with a band pass filter is attached to the lasersystem. The computer of the laser system monitors the dependence of thesignal on focal point depth. A change in the signal indicates theinterface between the lower surface of the aplanation lens and cornea.Accuracy of better than 5 microns may be achieved.

Referring to FIG. 6, the method may be further described. FIG. 6 is agraph illustrating dependence of second harmonic signal on beam waistposition in pig eye where the positive sign on the Depth axiscorresponds to the position inside the cornea and the zero positioncorresponds to the cornea-glass interface. To determine the focal pointof the laser beam at the interface of the aplanation lens and thecornea, one takes half the max of the signal. This is shown on the graphon at the point of 0 microns. If the focal spot moves out into theaplanation lens, then the signal decreases, if the focal point goes intothe cornea, then the signal increases. This can be done because, withcertain laser beams, such as a femtosecond mode-locked laser beam can bedescribed by its confocal parameter. In other words, the laser beam hasa focal point with a particular length range. It is when half the lengthof the focal point range is inside the cornea that the signal would beat the half max of the signal.

In one experiment, the method was tested with an aplanation lens incontact with a pig eye. The energy level of laser was reduced to 0.2 □Jso that the fluence is below the optical damage threshold of the glassor pig eye, but high enough to generate second harmonic in cornea. Whilescanning the depth of the focal point, the intensity of second harmonicdecreases by factor of 50 from cornea to glass interface. This enabledlocalization of the focal point at the cornea-glass interface withaccuracy of better than +/−5.0 microns. Results are presented on FIG. 6

In another experiment, the method was tested with an aplanation lenshaving a piece of plastic attached to the lens. The piece of plastic wasused to simulate a cornea being in contact with the aplanation lens. Theenergy level of the laser system was reduced to 0.7 μJ so that thefluence is below the optical damage of the glass, but high enough togenerate white light. While scanning the depth of the focal point, theintensity of while light changes by factor of two from glass to plastic.This enable the localization of the focal spot position at theglass-plastic interference with an accuracy of 5 micron.

The inventive systems and methods described above are well adapted for asystem to determine the position and alignment of an aplanation lens inrelation to a laser system. However, it shall be noted that theforegoing description is presented for purposes of illustration anddescription, and is not intended to limit the invention to the formdisclosed herein. Consequently, variations and modifications to thesystems and processes commensurate with the above teachings and teachingof the relevant art are within the scope of the invention. Thesevariations will readily suggest themselves to those skilled in therelevant art and are encompassed within the spirit of the invention andthe scope of the following claims.

Moreover, the embodiments described are further intended to explain thebest modes for practicing the invention, and to enable others skilled inthe art to utilize the invention in such, or other, embodiments and withvarious modifications required by the particular applications or uses ofthe present invention. It is intended that the appending claims beconstrued to included alternative embodiments to the extent that it ispermitted by the prior art.

1. A method for determining the focus of a laser beam about a surface ofan object, the method comprising the steps of: providing an objecthaving a substantially planar surface; providing a laser system forgenerating a laser beam; focusing a laser beam at or near thesubstantially planar surface, wherein the laser beam is reflected backfrom the planar surface; detecting a fringe pattern of the laser beam;and determining whether the laser beam is in focus.
 2. The method ofclaim 1, wherein the detecting step includes: providing aninterferometer; and utilizing the interferometer to analyze the fringepattern.
 3. The method of claim 1, wherein the determining stepincludes: evaluating the fringe pattern to determine if lines of thefringe pattern are substantially parallel to one another.
 4. The methodof claim 1, wherein the determining step includes: evaluating the fringepattern to determine the curvature of the lines.
 5. A laser system fordetermining the focus of a laser beam about a surface of an object, thesystem comprising: a laser system for generating a laser beam, the lasersystem having a central processing unit, the central processing unitconfigured for instructing movement of the laser beam; an interferometerfor detecting a fringe pattern, the interferometer interconnected withthe laser system; and a software program for execution on the centralprocessing unit, the software program configured for: focusing a laserbeam at or near the substantially planar surface, wherein the laser beamis reflected back from the planar surface; detecting a fringe pattern ofthe laser beam; and determining whether the laser beam is in focus. 6.The laser system of claim 5, wherein the determining step includes:evaluating the fringe pattern to determine if lines of the fringepattern are substantially parallel to one another.
 7. The laser systemof claim 5, wherein the determining step includes: evaluating the fringepattern to determine the curvature of the lines.